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Aug. 25, 2011 -- Steve Jobs gave no specific explanation for his sudden resignation as Apple CEO. But one possible health reason is that his pancreatic cancer may have returned.

If Jobs had suffered the most common form of pancreatic cancer, adenocarcinoma, the chances are he would have died soon after his 2003 diagnosis. But as Jobs later revealed, he had an unusual form of pancreatic cancer known as a neuroendocrine tumor or islet cell carcinoma.

In 2004, nine months after his diagnosis, Jobs underwent surgery to remove the tumor. In 2009 he underwent a liver transplant, a procedure appropriate for only a small number of patients with this uncommon form of pancreatic cancer.

What is known about this kind of cancer? Can it be cured? What if it comes back? WebMD answers these and other questions.

What Is a Neuroendocrine Tumor/Islet Cell Carcinoma?

When doctors discover that a patient has pancreatic cancer, the outlook usually is grim. But once in a while -- about 200 to 1,000 times a year in the U.S. -- it turns out to be an islet cell carcinoma.

Islet cells are the hormone-producing cells of the pancreas. It's no walk in the park to be diagnosed with cancer of these cells. But these cancers include "a highly treatable and often curable collection of tumors," according to the National Cancer Institute.

The course of disease depends on which of these cells become cancerous. Sometimes, as the tumor cells grow in number, they emit various hormones. This can have weird results, such as the inability to digest fats or sudden growth of the hands or feet. These hormone-emitting tumors often are benign.

Sometimes islet cell tumors don't make hormones. This avoids those bizarre effects. But 90% of these tumors are malignant, meaning that they eventually are fatal if left untreated.

Can Neuroendocrine Tumors/Islet Cell Carcinomas Be Cured?

The first choice of treatment for islet cell carcinoma is surgery, says David Levi, MD, professor of clinical surgery at the University of Miami Miller School of Medicine. Levi did not treat Jobs or have access to his medical records. His comments are about islet cell carcinoma in general and not specifically about Jobs' case.

"If it can be cured with surgery we try for that," Levi says. "If not there are options: chemotherapy and a number of other options to try to control this tumor. Some of these cancers are not curable, but patients can do well for years and years. ... Many can be treated medically for months and years and do quite well and lead normal lives to the last."

Jobs is said to have undergone the Whipple procedure. This is the preferred type of surgery when an islet cell tumor is on the head of the pancreas. It means that the head of the pancreas is removed, as is part of the bile duct, the gallbladder, and the first part of the small intestine. Sometimes part of the stomach is removed as well. Then the remaining parts of these organs are connected back to the small intestine.

Why Did Steve Jobs Have a Liver Transplant?

Although it's now known that Jobs received a liver transplant in Tennessee, it's not clear why. However, Levi says that a small number of patients with islet cell carcinoma may undergo liver transplant if their cancer has spread to the liver but does not appear to have spread elsewhere.

"A very small subset of patients with metastatic neuroendocrine tumors can be treated with liver transplant -- and this can be curative," Levi says.

It's not a procedure for people who may have cancer remaining in their bodies. That's because transplant patients must stay on immune-suppressing anti-rejection drugs for the rest of their lives. Without a functioning immune system, remaining cancer cells grow uncontrollably.

"The cancer can recur after liver transplant. When it does recur, it carries a pretty poor prognosis and ultimately is the cause of death," Levi says." We are limited in what we can do. The patient is immune suppressed. This usually means the cancer is aggressive, and once it recurs it usually is not curable."